The present invention relates to the measurement of force for industrial applications. It concerns the use of load cells for continuous measurement of forces such as weight or pressure, including the measurement of tension in web or sheet materials. More specifically the Invention is a new load cell.
Within many industrial areas it is necessary to measure accurately the magnitude of a force. An example of a load cell for the measurement of a force in a vertical direction is known. A load cell is arranged with two beams that are substantially thinner than the remainder of the load cell. Each of the beams has a measuring zone equipped with a measuring means. As a result of a vertical load F on the load cell a shear force due to F/2 is applied to the beam. Force in the measuring zone of the beams as a result of a vertical load is measured by a magnetoelastic sensor of the Pressductor type.
This load cell has a long service life, tolerates overloads well and performs accurately in service. This load cell requires a certain minimum load in order to generate a useable measurement signal, and its accuracy can be affected by temperature gradients across the load cell.
The object of the invention is to provide a load cell that measures loads accurately. A further object of the invention is to provide a load cell that measures small loads accurately. A still further object of the invention is to provide a load cell in which the effects of temperature gradients across the load cell are minimized.
A load cell according to the invention comprises a structural load-carrying framework of beams and joints arranged with one or more measuring zones. The structural load-carrying framework is arranged so that a load applied to two parallel outer beams of the framework is transferred to the measuring zones via connecting beams. At least one connecting beam is arranged at an angle xcex1 to the two parallel outer beams. The load applied to the structural load-carrying framework is transferred to the measuring zone as a load which is greater, smaller or the same as the applied load, in proportion to the angle xcex1.
A load cell according a preferred embodiment of the present invention is shaped externally as a solid and substantially rectangular block. Inside the substantially rectangular block the load cell is further shaped such that the load on two parallel outer beams of the load cell is applied to two measuring beams equipped with measuring zones arranged in the load cell and joined together by a membrane. Each of the measuring beams equipped with measuring zones is subjected to a tensile or compressive load in the direction of the long axis of the beam. The measuring beams are positioned on either side of a membrane in a preferred embodiment of the load cell.
The load transferred to each measuring beam in the area where the measuring zone is arranged is greater, smaller or the same as the external load on that part of the load cell. Through a type of leverage action, relatively small loads may be measured. The leverage is in proportion to the angle xcex1 between connecting beams in the load cell and a long axis of parallel outer beams arranged in the framework. A measuring means is arranged on each measuring zone to generate a signal proportional to the mechanical strain imposed on the measuring zone. A preferred embodiment of the invention comprises two measuring beams on either side of a membrane where a measuring signal is taken from a measuring zone arranged on each beam.
The advantage of the present invention is that the shape and geometry of the load cell makes it sensitive to small loads in a y-direction. A further advantage is that the two beams arranged with measuring zones are joined and arranged with an asymmetric geometry that makes the load cell insensitive to temperature gradients in two directions, one direction parallel to the long axis of the beam, the x-direction, and another direction perpendicular to it. Temperature gradients across the load cell in a straight line in the y-direction cause changes in signal from measuring zones on the two beams which cancel each other out. The same is true for temperature gradients across the load cell in the x-direction.
A still further advantage of the invention is that extension under load in the direction perpendicular to the long axes of the parallel outer beams is extremely small. A yet still further advantage includes that a given size of load cell according to the invention may thus be used to measure a wider range of loads. An additional further advantage of the invention is that the shape and geometry makes the load cell insensitive to loads parallel to the long axis of the parallel outer beams.